Osimhen's price is too high for a team like Ac Milan I don't know where the idea comes from to say Ac Milan trying to hire Osimhen for the next season but as far as I know, other teams in Serie A can't have enough money to hire this player for the next season and even if Napoli gets some offer for Osimhen from other teams, there will other teams like PSG with better financial situation.
But regardless of that, I think Napoli's first priority is to keep Osimhen for another season.
Osimhen's current market value is €120m, but I assume Napoli set a minimum transfer fees around €150m. It is surely a huge amount of money, only big clubs like Real Madrid or PSG that can pay the transfer fees. I also doubt that AC Milan or Inter Milan is interested to sign Osimhen, they may prefer to sign cheaper strikers. There are still many other strikers who have cheaper prices although the prices of most players are getting higher right now.
We don't know what the plan for Osimhen, but I think Napoli is better to sell him as soon as possible. I am not sure Napoli still can perform as well as the last season, Osimhen price will decrease if Napoli performance is decreasing. Unless getting a bigger money in the next transfer window, the top clubs may be not interested anymore with Osimhen. Napoli must let him leaving in the current transfer window if they want sell him at the best price.